Field of the Invention
The present invention relates to a nitride semiconductor substrate suitably used for the power semiconductor, electronic devices, etc.
Description of the Related Art
As the technique of providing the crystal growth method for a nitride semiconductor for improving the performance and the lifetime of the semiconductor device by reducing the dislocation density, JP-A-2002-323733 has disclosed the crystal growth method for a nitride semiconductor, the method including a first crystal growth step of forming a plurality of island-shaped crystal regions of nitride semiconductor on a base through vapor-phase growth, and a second crystal growth step of growing the island-shaped crystal regions further while linking the borders of the island-shaped crystal regions. In this method, the crystal growth speed in the second crystal growth step is set higher than the crystal growth speed in the first crystal growth step, or the crystal growth temperature in the second crystal growth step is set lower than the crystal growth temperature in the first crystal growth step. This enables the dislocation to be bent in the border linking portion so that the dislocation density can be decreased. The mechanism of the formation of a nitride layer when the nitride semiconductor layer is formed on the base substrate through the vapor-phase growth is a public knowledge.
JP-A-2013-087012 has disclosed the substrate for the nitride semiconductor growth, which enables to grow the nitride semiconductor with the low dislocation density. The substrate is a sapphire substrate whose main plane is the C-plane. On the main plane, conical or truncated projections having a side surface tilted by less than 90° relative to the main plane are formed in the lattice form. The height of the projection from the main plane is 0.5 μm or more and 3 μm or less, and the distance between the adjacent projections is 1 μm or more and 6 μm or less. The surface roughness RMS of the side surface of the projection is 10 nm or less. That is to say, it has also been known that the uneven shape of the one principal plane of the base substrate has an influence on the characteristics of the nitride semiconductor formed thereon.
The invention according to JP-A-2002-323733 is to decrease the dislocation of the nitride semiconductor layer by optimizing the growth condition for the nitride semiconductor layer. If the uniform growth of the nitride semiconductor layer on the entire surface of the substrate is failed, the high-quality nitride semiconductor layer may not be obtained.
The invention according to JP-A-2013-087012 is to eliminate the dislocation by making the base substrate uneven in advance to optimize the growth speed of the nitride semiconductor layer. However, forming the unevenness of such a size within the surface of the substrate with high accuracy results in the cost increase of the base substrate. Moreover, it is necessary to form the initial nitride semiconductor layer to such a degree that the unevenness is fully covered up, and this increases the cost of manufacturing the nitride semiconductor layer. If the unevenness is not formed uniformly, the nitride semiconductor layer to be formed thereon is possibly non-uniform.
That is to say, it is necessary that the uneven shape on one principal plane of the base substrate should be formed with the uniform and appropriate size. In addition, the uneven shape is desirably formed at the industrially low cost and with high accuracy.